In the interest of a better understanding of the invention, certain conventions and practices within the trade may be appreciated.
Lenticular lenses, or lenticules, are typically cylindrical bodies having longitudinal axes and arranged in a parallel-axis array on a lenticulated face of a lenticular sheet. The face opposite the lenticulated face typically is substantially planar. Seen in cross-section, each lenticule has a vertex distal from the planar face, and adjacent lenticules intersect to define valleys proximal to the planar face. A lenticular height is defined between a first plane tangent to the lenticule vertices and a second plane tangent to the lenticule valleys. A lenticular pitch is defined between axes of adjacent lenticules along a raster axis perpendicular to the axes of the parallel lenticules.
The lenticular sheet typically is formed with a thickness of the sheet being substantially equal to a focal length of the cylindrical lenses or lenticules. The graphic resolution along the raster axis is then limited to the lenticular pitch. In the finished lenticular product, the planar face commonly carries a specially prepared and registered printed image. The image is most usually printed directly to the planar face of the sheet, but may also be formed on a separate substrate, and then aligned and adhered to the planar face.
A tradeoff between quality of focus and viewing angle is well known in the lenticular art. The influence of refractive index is also well understood. Lenticular sheets are often described according to the lenticular pitch in lenses per inch. A 150 lens-per-inch (LPI) array is colloquially understood to be a fine pitch. 75 LPI lens is considered an industry standard. A 40 LPI lens has a relatively coarse pitch, generally used for applications in which the lenticular item is to be viewed at greater than arm's length. The majority of commercial applications are currently served by lenticular sheets having proportions between 1.2 times as thick as the lenticular height, to twice as thick as the lenticular height.
A single lenticule of a 75 LPI lenticular sheet is about 339 microns (13 mils) wide from valley to valley. In its most common present commercial form, a 75 LPI lenticular sheet will have a refractive index of around 1.57 and a thickness of around 469 microns (18 mils), therefore being about 1.4 times as thick as the nominal lens width.
It may be understood that some applications have called for more extreme proportions, as when a thin, conformable lenticular label is required, in which case the proportion may be 1:1 or less. Conversely, superior optical resolving power is often sought after in autostereoscopic “3-D” display, and in this case the ratio of thickness to lens width may be 3:1 or greater. The preceding values descriptions are intended to characterize underlying principles, and identify the most readily available commercial materials in the current trade, and should not in any way be taken to limit the scope of the invention.
Lenticular sheets may be formed by any suitable method. For example, U.S. Pat. Nos. 5,330,799 and 5,554,532 to Sandor et al. describe a lenticular system in which lenses are formed upon a flat carrier sheet in a forming process which is commonly known as “cast film” lenticular. and or et al. describe lenses formed in local areas by forming and curing fluid material over the desired image areas.
However, the cast film process has proven costly and has not been widely adopted. Instead, high-speed extrusion is currently the prevalent practice in the trade. Extrudable polymer materials suitable for use within the invention include amorphous polyethylene (APET), or glycol-modified polyethylene terephthalate copolyester (PETG). Additionally, formulations of polycarbonate, acrylic, styrene, and other polymers can also be used to form the prefabricated lens array by extrusion. It is recognized that such thermoplastic polymer materials are also generally amenable to other manufacturing methods, such as embossing or various other molding and forming techniques.
High-speed extrusion is cost-efficient for large runs, but requires forming cylinders that are individually expensive. Thus, the extrusion method is not economically feasible for making customized layouts of lenticulated surfaces in any arbitrary combination with unlenticulated regions. Nevertheless, there are many applications for lenticular lenses in which an ability to provide such customized layouts would be advantageous. Although alternate manufacturing methods such as cast film and injection molding may be used to prefabricate a patterned combination of lenticulated surfaces and smooth optical windows, such methods increase the cost of the finished product above what is economically practicable.
In the practice of lenticular printing, it has been discovered that the ribbed cylindrical relief inhibits the accomplishment of certain common and regularly sought-after tasks within the field of printed graphics. For example, the optical effect of the lenticular overlay has a disadvantageous effect upon the accurate detection of encoded data. The material currently having the widest use in the trade has 75 cylindrical lenses per inch (LPI). This pitch is significantly broader than the resolution of conventional offset printing. Indeed, a human reader cannot discern fine text that has been visually expressed via the lenticular overlay, as the maximum resolution on one axis is characteristically constrained by the lens pitch. Analogously, in many cases bar codes cannot be correctly read, owing to distortions imparted by the many cylindrical lenses. Thus, it may be appreciated that it might be desirable to make text, graphics or other indicia visible at a higher linear resolution than can be achieved by viewing a printed image through the surface optics of a lenticular lens sheet.
A particular problem identified in the lenticular trade has been the method's historical incompatibility with bar coding. The fine lines used in the encoded data often exceed the resolution available in standardized lenticulated sheet. Prior solutions to this specific problem include the use of 150 LPI or finer lenses, as suggested in U.S. Pat. No. 6,424,467 to Goggins, or disposing the bar code so that the bars in the barcode are oriented in a crosswise direction, as in U.S. Pat. No. 6,974,080, also to Goggins.
However, in the invention described in U.S. Pat. No. 6,424,467, such fine lenses are difficult to cost-effectively manage in a production environment. Furthermore, for a given optical design, the pitch of the lenticules and the thickness of the sheet are directly proportional. Therefore, end users often avoid lenses thinner than 75 LPI because, unless mounted in a frame or on a rigid substrate, such thin lenses are widely believed to be prone to unwanted flexure, a property which is known to disrupt the lenticular effect.
In U.S. Pat. No. 6,974,080, the magnification effects of the lenses are obviated only if the scanning device is held absolutely perpendicular to the lens sheet. It may therefore be appreciated that there remains a general desire both for more flexibility in the local optical features and resolution properties of lenticular sheet, and more particularly for improving the rate and reliability of the scanning of machine-readable optical indicia, such as bar codes.